1. Field of the Invention
The present invention relates to three-dimensional beamforming in a mobile communications network. In particular, the invention relates to acquiring channel information for three-dimensional beamforming in a mobile communications network.
2. Related background Art
The following meanings for the abbreviations used in this specification apply:    3-D three-dimensional    AAS active antenna system    CQI channel quality indicator    CSI channel state information    eNB evolved node B    HSPA high speed packet access    LTE long-term evolution    PHY physical layer    PMI preceding matrix indicator    RI rank indicator    RRM radio resource management    RS reference signal    SON self organizing network    UE user equipment    VoIP voice over internet protocol
The uplink feedback channel usually is a bottleneck for system performance in a mobile communications network. Efficient feedback schemes are needed to cope with an increased quantity of channel information with respect to lower resource usage and finer granularity of CSI knowledge at a base station, e.g. an eNB, of an access network of the mobile communications network. In this respect, introduction of UE-specific beamforming is a challenge to optimize feedback design.
The active antenna system (AAS) as a part of SON has provided significant gain effects by the use of optimized cell-specific downtilting. With the evolution of AAS, 3-D beamforming can be supported with either UE-specific or cell-specific beamforming, which is assumed to provide even greater gain effects.
Current 2-D beamforming creates horizontal beam oriented specific UEs. Methods to calculate a beamforming weighting vector are rather different among different candidate schemes, and a major implementation is an uplink channel estimation or feedback from UEs to obtain a channel response matrix and a downlink beam weighting vector calculation. The channel response matrix calculation costs lots of baseband processing resources and introduces delay, since the matrix has a size of NR×NT, where NR is the number of receiver antenna elements at a UE and NT is the number of transmitter antenna elements at an eNB in the horizontal domain.
When AAS with 3-D beamforming is introduced, the channel response matrix has a size of NR×NT_Horizontal×NT_Vertical, where NT_Horizontal is the number of horizontal transmitter antenna elements and NT_Vertical is the number of vertical transmitter antenna elements. In this case, the calculation of a 3-D beamforming weighting vector is very complex.